1. Field of the Invention
This invention relates generally to the field of electrical energy conversion systems and more particularly to push-pull inverter circuits utilizing a solid state active element oscillator of the multivibrator type to convert an input DC voltage to a high frequency AC output voltage.
2. Description of the Prior Art
Push-pull inverter circuits are generally recognized as the most efficient type for converting DC voltage into an AC output voltage. Such circuits typically include a source of DC potential, an output transformer, and a pair of switching transistors connected to control the flow of current through the output transformer for thereby producing an AC voltage output across the transformer. Efficient conversion of the DC voltage into the AC output voltage requires that the conduction of the switching transistors be precisely controlled. Such precise control can serve to minimize undesirable energy losses within the circuit itself. Some of the causes of such energy losses have been recognized and are generally regarded as inherent in such circuits, or in the components making up such circuits. Some of these losses are:
1. Common-mode conduction which occurs when both of the switching transistors conduct simultaneously. This loss is usually related to the inherent and generally unavoidable delay associated with the turn-off action of the conducting transistor, coupled with the fact that there generally is no corresponding delay associated with the turning on of the other transistor.
2. Turn-off transition loss which is due to the power dissipation that occurs within each transistor during its turn-off transition. To minimize this loss, it is necessary to operate each transistor near its maximum switching speed capability. This in turn requires that the charge carriers stored at the transistor base-emitter junction be evacuated as rapidly as possible.
It is also more important to prevent the collector voltage from rising significantly before the transistor has been turned off completely.
3. Premature turn-on. The energy dissipation due to this factor results from turning on a transistor before its collector voltage has been reduced to its minimum level.
4. Dissipation while turned on. Significant energy loss can result from power dissipation within each transistor during normal forward conduction. To minimize this loss, it is necessary to provide adequate but not excessive base drive corresponding to the collector current flowing at any given time. If more than just adequate base drive is provided, this drive can itself constitute a cause of excessive energy dissipation.
The causes of energy loss enumerated above and the consequent reduction in efficiency in inverter circuits have been recognized by others to a limited extent. The patent art also teaches some proposed solutions to correct for some of the identified energy losses. An example of such recognition is to be found in the patent to Hook, U.S. Pat. No. 3,913,036, issued on Oct. 14, 1975. This patent is directed to a saturable core astable multivibrator circuit for providing a square wave AC output voltage from a relatively low DC voltage power source. Hook's circuit includes two alternately conducting high power transistors for delivering an AC output voltage to a transformer. The conduction of the transistors is controlled by a base drive circuit containing a saturable core transformer and a pair of diodes each connected across the base to emitter junction of the transistors. The operation of Hook's circuit is dependent upon the transistors which are characterized as having significant parasitic junction capacitance with respect to the diodes which have relatively slow forward and reverse recovery time characteristics. The combination of these characteristics is utilized, purportedly, to effect non-conduction of one transistor before the other is switched on and thereby minimize common mode conduction and the consequent power dissipation.
A number of other inverter circuits are also described in the patent art and utilize separate drive or oscillator circuits for controlling the switching on and off of the transistors. Such drive circuits and to the total cost and can be wasteful of energy unless closely matched to the characteristics of the inverter circuit per se.
Alternative solutions to the enumerated causes of energy loss have been proposed in my pending applications entitled:
High Efficiency Push-Pull Inverters (filed Mar. 20, 1978, Ser. No. 890,586), and PA1 High Efficiency Inverter and Ballast Circuits (filed Dec. 28, 1978, Ser. No. 973,741).